The frequency-controlled a.c. motor is the most advanced design for controlled speed drives, e.g. in elevators. With frequency control the efficiency is constantly high at any motor speed, and the mains power factor is nearly unity. Frequency control is applicable in gearless elevators and in elevators with gear transmission, and with any speed. Moreover, it is possible to use for the motor a simple and reasonably priced squirrel cage motor. In elevator applications, a transistor inverter implemented with transistors is best suited for use in frequency control because transistors enable the highest switching frequency to be achieved, among presently existing power electronics components. GTO thyristors may also be contemplated, but their use is somewhat more cumbersome than that of transistors. This is due to the components required for the switching protection circuits which have to be used with GTO thyristors.
Experience has revealed that a good strategy for controlling a squirrel cage motor is to keep the magnetic flux constant. As a rule, inertia is associated with magnetic flux. If the magnetic flux changes, a time constant is introduced in the system. Stabilizing of the magnetic flux may be implemented by maintaining the magnetic flux in the stator or in the rotor or in the air gap constant. Most advantageous is the procedure in which the magnetic flux of the stator is kept constant because the risk of saturation of the components in the motor's magnetic circuit is then at its lowest.
It is present practice, in forming the reference voltage for an inverter, to use current feedback in which the current drawn from the inverter output is measured. The drawback of current feedback is slow response. This slow response is caused by the fact that in current feedback the inductances of the motor give rise to time constants. The control loop, as a rule, becomes slower as the number of time constants in it increases. Moreover, current measuring elements are expensive because they must also be able to measure direct current.